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Simply so, what is better shellac or polyurethane?

It is extremely durable and one of the hardest of the topcoat finishes. It is stronger and better wearing than shellac and varnish. (Debatable as to whether or not it's harder and more durable than polyurethane).

Secondly, can I put shellac over polyurethane? Simply put, shellac covers all. Working the other direction or when you apply other topcoats over shellac, it is often noted that you should not apply polyurethane over shellac. It is, however, OK to use an oil-based urethane over shellac.

Correspondingly, what is the difference between varnish shellac and polyurethane?

The terms varnish, shellac, lacquer and polyurethane are commonly used to refer to the final finishes we are talking about. It can then be used as a brush-on colorant and most commonly as wood finish. Varnish, on the other hand, is a protective finish or film that is very hard and is also used in wood finishing.

What is shellac used for?

It is processed and sold as dry flakes (pictured) and dissolved in alcohol to make liquid shellac, which is used as a brush-on colorant, food glaze and wood finish. Shellac functions as a tough natural primer, sanding sealant, tannin-blocker, odour-blocker, stain, and high-gloss varnish.